1. Field of the Invention
The present invention relates to an optical head for a magneto-optical information reproducing apparatus for reproducing information magnetically recorded on a magneto-optical recording medium by utilizing a magneto-optical effect.
2. Related Background Art
The study and development of an optical memory for recording and reproducing information by a semiconductor laser beam for use as a high record density memory have recently been vigorously done, and particularly a magneto-optical recording medium which permits erasing and rewriting of information is considered promising. In the magneto-optical recording medium, information is magnetically recorded by utilizing a local temperature rise on a magnetic thin film by spot irradiation of a laser beam, and the information is reproduced by a magneto-optical effect (particularly, a Kerr effect). The Kerr effect is defined as a phenomenon in which a polarization plane is rotated when light is reflected by a magnetic recording medium.
In a prior art optical head for the magneto-optical reproducing apparatus, a construction of an optical head which uses a crystal is explained as means for reproducing the information.
In an optical head 101 shown in FIG. 1, a light beam from a semiconductor laser 102 is collimated by a collimator lens 103, reflected by a beam splitter 104, and focused by an objective lens 105 onto a magneto-optical disk 106 which is a magneto-optical recording medium. The light beam reflected by the magneto-optical disk 106 passes through the objective lens 105 and the beam splitter 104 and is split by a beam splitter 107 into two light beams, a reflected beam and a transmitted beam. The reflected beam is directed to a servo error detection photo-sensor 109 through a lens 108. The photo-sensor 109 produces a detection signal in accordance with a shape of a spot on a photo-sensing area and supplies it to a servo error signal generator 112, which in turn produces a focusing error signal and a tracking error signal which are used to drive the objective lens 105 to a desired position by an actuator (not shown).
On the other hand, the light beam transmitted through the beam splitter 107 is directed to a one-half wavelength plate 111, split by a wallastone prism 113 into two light beams having orthogonal polarization components, and they are directed to a photo-detector 115 through a lens 114. The photo-detector 115 has two detectors for the above two light beams, and produces two detection output signals representing the changes in the respective polarization components and supplies them to a reproduced signal detector 116, which compares the two detection output signals to detect a rotation (Kerr rotation of the polarization plane of the light beam caused in the reflection by the vertically magnetized film of the magneto-optical disk 106 so that a reproduction signal representing the rotation is produced.
Referring to FIGS. 2 and 3, a principle of the two-beam split by the wallastone prism 113 is explained. The wallastone prism 113 is constructed by joining a quartz 120 having an optical axis 130 parallel to a Y-axis and a quartz 121 having an optical axis 131 parallel to a Z-axis. An incident light which travels along an X-axis is a linearly polarized light which is polarized in an XY plane. The one-half wavelength plate 111 is set to rotate the polarization direction of the incident light by 45 degrees.
FIG. 3 shows a sectional view of the prior art wallastone prism 113 shown in FIG. 2. When the incident light polarized in the direction of 45 degrees with respect to the Y-axis travels from the quartz 120 to the quartz 121, a projection component to the Y-axis is sequentially subjected to the actions of an extraordinary light refractive index and an ordinary light refractive index and exits as a linearly polarized light 122 polarized in the XY-plane. On the other hand, a projection component of the incident light to the Z-axis is sequentially subjected to the actions of the ordinary light refractive index and the extraordinary light refractive index, and exits as a linearly polarized light 123 which is polarized in a plane orthogonal to the XY plane. Thus, the incident light which is polarized in the direction of 45 degrees with respect to the Y-axis exits as the two linearly polarized lights 122 and 123 which are orthogonal to each other and have the same intensity. The polarization direction of the light beam reflected by the magneto-optical disk 106, prior to the entrance into the one-half wavelength plate 111 is deviated from the Y-axis by .theta..sub.K or -.theta..sub.K due to the influence of the Kerr effect. When the polarization direction of the incident light periodically changes between the direction corresponding to .theta..sub.K and the direction corresponding to -.theta..sub.K, the two exit lights 122 and 123 have the same amplitude and opposite phases. Accordingly, by differentiating the outputs from the detectors for the two exit lights, a light intensity variation (noise) due to a foreign material on the magneto-optical disk is illuminated and a C/N ratio of the reproduced signal is improved.
However, in the above example, the following problems arise. First, the beam splitter 107 is required to produce the light beam for generating the servo signal and the light beam for generating the reproduction signal. Secondly, the one-half wavelength plate 111 is required to set the desired polarization direction of the light beam applied to the wallastone prism 113. Thirdly, a large number of steps are required for the adjustment of the angle of the one-half wavelength plate 111.